Variable pitch corrugated waveguide



Oct. 3, 1967 K. WOLFGANG ETAL VARIABLE PITCH CORRUGATED WAVEGUIDE 2 Sheets-Sheet 1 Filed Feb. 27, 1967 FIG.1

FIGS

m wa m o N T m R mm o w; w l .m .JIA m PW? United States Patent H i 9 Claims. (Cl. 333-95) This application is a continuation-in-part of copending application Ser. No. 409,485, filed Nov. 6, 1964.

Waveguides formed of tubular members have become well known. In this connection, tubular members of circular cross section as well as members with cross sections other than circular, preferably those with elliptical cross sections have come under consideration. For mechanical advantages as well as for electrical reasons, it has turned out to be desirable to make the tubing with corrugated or ridged walls. In a preferred embodiment, the corrugated or ridged walls have their undulations extending about the tube periphery in helical form. It is also already known that the sense of the helical corrugations can vary between two limits once or several times along the length of the pipe.

The present invention relates to a waveguide construc tion, having a preferably circular or elliptical cross section, where the corrugations run helically along the periphery of the tube and where the sense of the pitch of the helix changes along the length of the pipe either once or several times. The waveguide according to the present invention differs from that previously known in that over a given length of the tube extending to a point i where the sense of the helical pitch changes, the pitch of the helical corrugations is reduced, as that point is approached, to the value of zero, either continuously or in stepwise fashion.

With a waveguide of the type cited above, that is with a waveguide consisting of corrugated or ridge-walled tubing there occurs a frequency-dependent rotation of the plane of polarization of the transmitted wave. It is already known that this frequency-dependent rotation of the plane of polarization can be compensated by reducing the depth of the corrugations over a portion of the length of the tubing to the value zero, either continuously or in stepwise fashion and to arrange the transverse plane at which the depth of the corrugation is zero, in such a position along the length of the guide, that the plane of polarization at the output is either the same as at the input or is shifted with respect to the input by 180. This can be accomplished either by locating this plane in the middle of the transmission line or by displacing the plane towards one end of the line. In the latter case, compensation occurs in the shorter portion of the line only to that extent to which the position of the plane of polarization in the longer portion of the line is turned out of the plane originally intended.

From the pointof view of fabrication, h-owever, it is preferable to keep the depth of the corrugations constant so that the present invention has the particular advantage that by selection of the transverse plane of the guide. at which the helical pitch is zero, one can also eifect a com pen-sation of the frequency-dependent rotation of the plane of, polarization without excessive additional efiort during fabrication. p V

To counteract the reflections which .occur with the variation of the pitch of the helical corrugations and to compensate these reflectionsat least in part, it is a special advantage when the length over which the. pitch. of the corrugations varies between the two limiting values of opposite sense, is an integral multiple of a quarter Waveice length M4 where is the mean waveguide wavelength of the band to be transmitted.

For longer lengths of waveguide it is necessary to compensate the frequency-dependent rotation of the plane of polarization in broadband fashion. When for this purpose, in accordance with the invention, the pitch of the helix along the guide wall is altered two or more times in the sense of the present invention, then it is an especial advantage to maintain, for the plane in which the helical pitch is zero, a maximum spacing which is dependent on the relative frequency bandwidth Af=(f :f )/f Where f denotes the mean frequency of the band to be transmitted and f denotes the upper or lower frequency limit of that band. Since the value of the pitch-angle influences the relative frequency bandwidth, the spacing is further dependent on the chosen pitch angle. This will be explained by way of example. Using a value f =4 go. (7.5 cm.) and a relative bandwidth of Af=0.1 with a pitch angle of 8 in that portion of the guide where the pitch is constant, the spacing between the planes described above should not exceed a value of 80%.

A waveguide constructed in accordance with the present invention is well suited for those transmission lines in which there occur one or more bends. There it is an advantage when the peak of the bend lies at least approximately in the plane where the pitch of the corrugations is zero. In this case, the region. of variable helical pitch will lie at least approximately symmetrically with respect to the point of peak bending. An even better accommodation to the curvature situation can be achieved if the length of the bend is at least approximately equal to the length over which the helix pitch changes from one extreme value to the corresponding value of opposite sense. One then achieves a condition where the electrically disturbed regions which exist in the region of the bend are practically identical with those that are produced by the change in helical pitch. One must in fact generally observe that through the compression and stretching of the guide wall in the region of the bend there exist, in the plane of the bend, diiferent lengths of conductor at the inside and at the outside of the bend.

One achieves almost complete compensation of the electrical irregularities caused by the compression and stretching of the guide walls if, in accordance with the invention, the change of pitch of the corrugations is determined as follows. The converging extensions of the legs of the different pitch angles in the yet as unbent pipe intersect in the intended plane of the bend at a point; this point is related to the center of the intended bending arc by mirror-imaging and translation. As a result of this procedure the mechanical compression of the inside of the bend does not result in a shortening of the electrical distance and the mechanical stretching at the outside of the bend does not result in elongation of the electrical distance; As a result, the electrical characteristics of the bent waveguide are at least approximately the same as that of a guide having no bends in the length thereof.

It is obvious that for the achievement of favorable electrical relationships one must strive to obtain a certain relationship between the mean waveguide wavelength and the spacing between those planes where the pitch of the corrugation is zero. Further, it turns out, in view of the above, that the sense of the helical pitch at the output end of a bend of certain sense can lead only into an input with a curvature of opposite sense if the advantage of the method described in the preceding, paragraph is not to be lost.

When there occurs, in a Waveguide having a circular or elliptical cross-section, several successive bends with similar or opposite sense, and where each bend can be up to about it is an advantage according to this invention, if the planes in which the helical pitch is zero, are spaced along the pipe axis by a minimum spacing l, which is given by the relationship:

where A denotes the mean waveguide wavelength of the band for which operation is intended and where 11 denotes an odd integer. In addition, there should be between two bends of the same sense always an odd number of such planes and between two bends of the same sense the number of such planes should be an even number or zero.

As far as possible, taking into account fabrication and installation techniques, it is recommended that the waveguide of this invention be in continuous tube form. If this is not possible, as because of installation factors, then it is advantageous to make the bends as special components with optional connecting cross-section. These components can be connected with each other conventionally, as by flanges, without deleterious effect.

In general when fabrication with a continuous tube is not feasible, it is advantageous to subdivide the guide in the plane where the helical pitch is zero, e.g. at the peaks of the bends and each portion of the guide is then made as a special component. In this manner, it becomes possible to accommodate also those bends which follow in a plane that turns in a direction opposite to that of the plane of curvature of the adjacent bend.

In the drawings,

FIG. 1 is a side elevational view of a waveguide construction embodying the invention;

FIG. 2 is a diagrammatic showing of the variations in helical pitch of the waveguide;

FIG. 3 is a curve showing the progressive change in pitch values of the helix of the waveguide;

FIG. 4 is a diagrammatic showing of pitch values for a waveguide construction which is bent to angular form;

FIG. 5 is a diagrammatic showing of the bent waveguide corresponding to the showing of FIG. 4;

FIG. 6 is a diagrammatic showing of the pitch values for a waveguide which will have successive bends; and

FIG. 7 is a diagrammatic showing of the bent waveguide corresponding to the showing of FIG. 6.

As shown in FIG. 1, 10 designates a waveguide embodying the invention, and more particularly, in the form of a corrugated tube whose inside diameter is D, and whose outside diameter is D The helical corrugated section 11 to the left of medial transverse plane 12 starts with a pitch having a maximum value of the pitch angle a, which pitch decreases progressively toward plane 12 where the pitch angle is zero. At the plane 12, and continuing in the right hand helically corrugated section 13, the sense of the direction of corrugation is reversed, and the angle of pitch progressively increases toward the outer right hand end of section 13 until it finally reaches a maximum value of the pitch angle a.

In FIGS. 2, 3, the variations in the helical pitch of a waveguide 10A are shown as reversing as to the sense of corrugation, several times along the length of the device indicated at Z. In the course of such changes of sense, there always occurs a point at which the value of the angle of pitch is equal to zero; as the transverse dashed lines 14, 15, 16 and 17. The maximum values for the angle of pitch as a or oc, is indicated at the dot-dash lines 18, 19 and 20.

At points corresponding to each of the lines 18, 19 and 20, it can be assumed that the guide 10A has been lengthened by inserting a section in which the pitch of the helix corrugations is constant at a value of at or o. This would correspond in FIG. 3 to inserting in the indicated sine wave, sections parallel to the Z-axis.

In order to compensate for the frequency-dependent rotation of the plane of polarization over as wide a band as possible, the maximum distance between the planes indicated by the lines 14, 15, 16, 17 is restricted to a certain maximum value. As noted above, this maximum value of the spacing is when the mean waveguide wavelength is 7.5 cm. (4 go.) with a relative bandwidth Af=0.l and a helical pitch of 8.

Further, the distance over which the helical pitch angle changes from a to --Ot, corresponding in FIG. 2 to the distance between two dot-dash lines 18, 19; 19, 20, is dimensioned to be an odd multiple of a quarter wavelength )\/4 where A is understood to be the mean waveguide wavelength of the band which it is intended to transmit. Each of the dimensioning rules indicated can be applied independently of the others as the first rule cited refers to the spacing of the planes where the pitch of the helix is zero, i.e. 04:0, while the second refers to the length of the zones where the value of the helix pitch is varying, that is where it is neither on or a.

In FIG. 4 is shown schematically a waveguide 25, where the pitch of the helix corrugation is placed on the straight tube such that the waveguide at one end can be bent to a desired angle which can have a value up to The converging legs of the pitch angles :a, and i-oc whose extensions are shown by lines P, intersect in the plane of curvature at a point m which is related to the point M in FIG. 5 by mirror imaging and translation. Point M in FIG. 5 is the center of the arc of curvature. In bending tube 25 to form tube 25A, the convergence of the legs of the pitch angles shown in FIG. 4 is reduced increasingly until the parallel condition is reached and then divergence occurs. The final state is reached when the extensions of the same legs in opposite directions (dashed lines Q, FIG. 5) intersect at the center of the arc of curvature, point M, and thus become radii of curvature. It should be noted that the pitch variation is tailored to a particular radius of curvature of the bend.

The desired radius of curvature of the bend is specified. A portion of the waveguide having this desired curvature is given, e.g. the showing in FIG. 5, guide 25A. The center of curvature of the bend of this portion of the guide is determine-d by basic geometry. The center of curvature is mirror-imaged and translated to obtain the point In, FIG. 4. Vectors are projected from point m to a straight line at the plane of reflection. Their angles of incidence at each point indicate the pitch angle required at the point.

Such a procedure can be carried out graphically or analytically. The required pitch angles are then used in corrugating a straight seam welded tube. The so corrugated tube can be rolled on a reel and stored or shipped. At the site of installation, the guide is unrolled and bent to have the preselected radius of curvature as indicated in FIG. 5. As long as this preselected radius of curvature can be maintained, bends, i.e. changes in direction of the guide, up to 90 can be obtained without affecting the electrical properties of the waveguide.

FIG. 6 shows a schematic section of a waveguide 30 which has been prepared for several bends. The helical pitch in the left section 31 is chosen so that the extensions of the legs of the pitch angles intersect at point m, while those of the right section 12 intersect at point In". As in the case of the showing of FIGS. 4, 5, these points correspond to the centers M, M" of the prospective curves shown in FIG. 7 for the bent waveguide 30A.

Here, as in FIG. 2, the dot-dash line 33 corresponds to the limit where the pitch angle has reached its maximum value or where it just begins to depart from that value. Here also, one can insert regions of constant pitch angle between regions where the pitch angle is variable. From FIGS. 6, 7 it follows that as explained above, an odd number of planes with zero helical pitch lie with advantage between bends of like sense while between bends of opposite sense there are no such planes (as in FIG. 7) or there are an even number of such planes.

As various changes might be made in the embodiments of the inventioin herein disclosed without departing from the spirit thereof, it is understood that all matter herein described or illustrated is not by Way of limitation except as set forth in the appended claims.

What is claimed is:

1. A waveguide comprising a tubular member of armate cross section, the wall of said member being formed with helical corrugations along the longitudinal extent thereof, the sense of rotation of said corrugations changing at least once in said longitudinal extent thereof, wherein the pitch of the helical corrugations progressively changes along a given longitudinal section thereof from a given maximum angular value at one end of said section to an angular value of zero at the other end of said section, the point at which said angular value is zero corresponding to the point at which the sense of rotation of said corrugations changes.

2. A waveguide as in claim 1 wherein the longitudinal extent of the waveguide which has a pitch angle of a given value at one end thereof, and a similar pitch angle of opposite sense at the other end thereof, is an integral multiple of M4 where A is the mean wavelength to be transmitted by the waveguide.

3. A waveguide as in claim 1 wherein at least two points at which said pitch angle is zero are located at a spacing which does not exceed a value such that the frequency-dependent rotation of the plane of polarization can "be broad band compensated.

4. A waveguide as in claim 1 wherein said guide is angularly bent, and the peak of said bend coincides with that point in the corrugation helix where the angle of pitch thereof is zero.

5. A waveguide as in claim 4 wherein the longitudinal extent of the bend is at least equal to the distance over which the pitch of the corrugation helix varies from one maximum value to an equal maximum value of opposite sense.

6. A waveguide as in claim 5 wherein, in the tubular member in its initial unbent form, the change in pitch of the corrugation helix is such that the converging projection of the pitch angle of the successive corrugations intersect at a point in the plane of curvature, and such point corresponds to the center of the arc of curvature of the waveguide in its bent form.

7. A waveguide in accordance with claim 1 wherein said tubular member has at least two successive angularly bent portions, each portion having a bend angle not exceeding the points in said member where the corrugation helix pitch is zero having a minimum spacing along the longitudinal axis of said member of the value of l where where designates the mean wavelength of the band to be transmitted and n is an odd integer.

8. A waveguide as in claim 7 wherein there is an odd number of said points between bends of the same sense. 9. A waveguide as in claim 7 wherein there is an even number of said points between bends of opposite sense.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner. L. ALLAHUT, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,345,590 October 3, 1967 Wolfgang Krank et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below In the drawings, sheets 1 and 2, line 1, for K. WOLFGANG ET AL", each occurrence r'ead W. KRANK ET AL in the heading to the printed specification, line 3, for "Krank Wolfgang" read Wolfgang Krank Signed and sealed this 22nd day of October 1968 (SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. A WAVEGUIDE COMPRISING A TUBULAR MEMBER OF ARCUATE CROSS SECTION, THE WALL OF SAID MEMBER BEING FORMED WITH HELICAL CORRUGATIONS ALONG THE LONGITUDINAL EXTENT THEREOF, THE SENSE OF ROTATION OF SAID CORRUGATIONS CHANGEING AT LEAST ONCE IN SAID LONGITUDINAL EXTENT THEREOF, WHEREIN THE PITCH OF THE HELICAL CORRUGATIONS PROGRESSIVELY CHANGES ALONG A GIVEN LONGITUDINAL SECTION THEREOF FROM A GIVEN MAXIMUM ANGULAR VALUE AT ONE END OF SAID SECTION TO AN ANGULAR VALUE OF ZERO AT THE OTHER END OF SAID SECTION, THE POINT AT WHICH SAID ANGULAR VALE IS ZERO CORRESPONDING TO THE POINT AT WHICH THE SENSE OF ROTATION OF SAID CORRUGATIONS CHANGES. 